Chemistry Department and Institute of Soft Matter, Georgetown University, 37th and O Streets, NW, Washington, DC, 20057-1227, USA.
Chemistry. 2019 Jan 7;25(2):400-416. doi: 10.1002/chem.201802385. Epub 2018 Nov 2.
A category of parallel π-stacking interaction, termed pancake bonding, is surveyed. The main characteristics are: the interaction occurs among radicals with highly delocalized π-electrons in their singly occupied molecular orbitals (SOMOs), the contact distances in the π-stacking direction are shorter than the typical van der Waals distances, and the stabilization obtained by the bonding combination of the SOMO orbitals leads to direct atom-to-atom overlap with strong orientational preferences. These atypical intermolecular interactions contain a component of electron sharing between the radicals that can be viewed as covalent-like. Pancake bonded dimers characteristically have low-lying singlet and triplet states and show characteristic interlayer vibrational modes. Pancake bonded aggregates serve as molecular components in many conducting and other functional organic materials. The role of van der Waals (vdW) interactions in pancake bonded dimers, chains, and other aggregates is different from closed shell vdW aggregates: here the Pauli repulsions reduce the attractive dispersion interaction significantly. Fluxionality between π- and σ-bonded aggregates often occur in the context of pancake bonding. Both experimental and computational aspects are reviewed.
本文综述了一类称为“煎饼键合”的平行π-堆积相互作用。其主要特点为：作用发生在具有单占据分子轨道（SOMO）中高度离域π-电子的自由基之间，π-堆积方向上的接触距离短于典型的范德华距离，由 SOMO 轨道键合组合获得的稳定化导致直接的原子-原子重叠，具有强烈的取向偏好。这些非典型的分子间相互作用包含自由基之间电子共享的成分，可以视为类似共价键的成分。煎饼键合二聚体具有低能的单重态和三重态，并表现出特征的层间振动模式。煎饼键合的聚集体作为许多导电和其他功能有机材料的分子组成部分。范德华（vdW）相互作用在煎饼键合二聚体、链和其他聚集体中的作用与封闭壳层 vdW 聚集体不同：这里的泡利排斥显著降低了吸引力的色散相互作用。在煎饼键合的背景下，π-键和σ-键合聚集体之间经常发生易位。本文综述了实验和计算方面的研究进展。
